ATP-sensitive K+ channels are involved in the mediation of intrathecal norepinephrine- or morphine-induced antinociception at the spinal level: a study using EMG planimetry of flexor reflex in rats

Glycogenolysis and purinergic signaling

Both ATP and glutamate are on one hand essential metabolites in brain and on the other serve a signaling function as transmitters. However, there is the major difference that the flux in the pathway producing transmitter glutamate is comparable to the rate of glucose metabolism in brain, whereas that producing transmitter ATP is orders of magnitude smaller than the metabolic turnover between ATP and ADP. Moreover, de novo glutamate production occurs exclusively in astrocytes, whereas transmitter ATP is produced both in neurons and astrocytes. This chapter deals only with ATP and exclusively with its formation and release in astrocytes, and it focuses on potential associations with glycogenolysis, which is known to be indispensable for the synthesis of glutamate. Glycogenolysis is dependent upon an increase in free intracellular Ca(2+) concentration (Ca(2+)]i). It can be further stimulated by cAMP, but in contrast to widespread beliefs, cAMP can on its own not induce glycogenolysis. Astrocytes generate ATP from accumulated adenosine, and this process does not seem to require glycogenolysis. A minor amount of the generated ATP is utilized as a transmitter, and its synthesis requires the presence of the mainly intracellular nucleoside transporter ENT3. Many transmitters as well as extracellular K(+) concentrations high enough to open the voltage-sensitive L-channels for Ca(2+) cause a release of transmitter ATP from astrocytes. Adenosine and ATP induce release of ATP by action at several different purinergic receptors. The release evoked by transmitters or elevated K(+) concentrations is abolished by DAB, an inhibitor of glycogenolysis.

The involvement of potassium channels in the peripheral antiedematogenic effect of intrathecally injected morphine in rats

BACKGROUND

A previous study indicated that intrathecal administration of morphine reduces experimental inflammatory edema in rats by activating the nitric oxide/cyclic guanosine monophosphate pathway. This evidence supports the hypothesis that potassium channel opening may play an important role in mediating morphine's effect under such conditions.

METHODS

Male Wistar rats received intrathecal injections of drugs (20 μL) 30 minutes before paw stimulation with carrageenan (150 µg). Edema was measured as paw volume increase (in milliliters), and plasma leakage was measured by Evans blue dye leakage. Neutrophil migration was evaluated indirectly by myeloperoxidase assay. The inflammatory infiltration and vascular congestion were observed by histologic examination.

RESULTS

Morphine (37 nmol) inhibited inflammatory edema, plasma leakage, and vascular congestion but had no effect on myeloperoxidase activity or neutrophil content compared with phosphate-buffered saline. Coinjection with 4-aminopyridine (10 nmol), glibenclamide (5 nmol), and dequalinium (10 pmol) reversed, but nicorandil (0.03 nmol) enhanced the effect of morphine.

CONCLUSIONS

These results support the hypothesis that the peripheral antiedematogenic effect produced by intrathecal morphine is mediated by potassium channel activation. Furthermore, this opioid effect does not involve the inhibition of acute neutrophil migration but does involve a reduction in capillary recruitment.

Sex differences in the contribution of ATP-sensitive K+ channels in trigeminal ganglia under an acute muscle pain condition

In this study, we examined whether functional subunits of the ATP-dependent K+ channel (KATP) are expressed in trigeminal ganglia (TG), which contains sensory neurons that innervate oral and facial structures. We also investigated whether direct activation of the KATP effectively attenuates mechanical hypersensitivity in the context of an acute orofacial muscle pain condition. The KATP expression in TG and behavioral studies were conducted in age matched male and female Sprague-Dawley rats. RT-PCR experiments showed that the mRNAs for the inwardly rectifying pore-forming subunits, Kir6.1 and Kir6.2, as well as the regulatory sulfonylurea subunits, SUR1 and SUR2, were reliably detected in TG. Subsequent western blot analysis confirmed that proteins for all four subunits are expressed in TG, and showed that Kir6.2 is expressed at a significantly higher level in male TG compared to that of female rats. This observation was confirmed by the immunohistochemical demonstration of higher percentages of Kir6 positive masseter afferents in female rats. Masseteric injection of capsaicin evokes a time dependent increase in masseter sensitivity to noxious mechanical stimulation. A specific KATP agonist, pinacidil, dose-dependently attenuated the capsaicin-induced mechanical hypersensitivity in male rats. The dose of pinacidil (20 μg) that completely blocked the capsaicin responses in male rats was ineffective in female rats regardless of their estrus phases. Only at the highest dose (300 μg) we used, pinacidil was partially effective in female rats. Similarly, another KATP agonist, diazoxide which targets different KATP subunits also showed sex specific responses in attenuating capsaicin-induced masseter hypersensitivity. These data suggested that sex differences in functional KATP expression in TG may underlie sex specific responses to KATP agonists. The present study provided novel information on sex differences in KATP expression in TG and its contribution under an orofacial muscle pain condition.

Deciphering the mechanism(s) of action of natural products: analgesic peroxide oil as example

BACKGROUND

There are multiple reports of natural products having therapeutic effect. In an era of evidence-based medicine, clinical trials inform clinical decisions regarding use of the product, but prevailing preference is to identify and use a single 'active ingredient'. Yet, the clinical benefit of a natural product might derive from the fortuitous combination of its multiple components. Therefore, the elucidation of the mechanism(s) of action of natural products is important, but presents significant challenges. This article examines this issue using peroxide oil (essential oxygen oil) as an illustrative example.

OBJECTIVE

To review the published literature of a natural product in an effort to elucidate postulated mechanism(s) of action of a complex mixture.

METHODS

The clinical and preclinical literature was reviewed from the perspective of its contribution to elucidating a mechanism of analgesic action of a natural product.

RESULTS

Peroxide oil contains ingredients that are associated with analgesic mechanisms, such inhibition of lipid peroxidation and arachidonic acid metabolism and non-opioid, glibenclamide-sensitive receptor-mediated and K(ATP) -NO-cGMP channel pathways.

CONCLUSION

Although its exact mechanism remains unknown, peroxide oil provides an example of how a natural product can be evaluated for plausible mechanistic explanation of its purported therapeutic efficacy. Such an approach seems valuable, since, as in this case, the constituents appear to contribute in an additive or synergistic manner, something not possible with a single substance.

Flexor reflex decreases during sympathetic stimulation in chronic human spinal cord injury

A better understanding of autonomic influence on motor reflex pathways in spinal cord injury is important to the clinical management of autonomic dysreflexia and spasticity in spinal cord injured patients. The purpose of this study was to examine the modulation of flexor reflex windup during episodes of induced sympathetic activity in chronic human spinal cord injury (SCI). We simultaneously measured peripheral vascular conductance and the windup of the flexor reflex in response to conditioning stimuli of electrocutaneous stimulation to the opposite leg and bladder percussion. Flexor reflexes were quantified using torque measurements of the response to a noxious electrical stimulus applied to the skin of the medial arch of the foot. Both bladder percussion and skin conditioning stimuli produced a reduction (43-67%) in the ankle and hip flexor torques (p<0.05) of the flexor reflex. This reduction was accompanied by a simultaneous reduction in vascular conductance, measured using venous plethysmography, with a time course that matched the flexor reflex depression. While there was an overall attenuation of the flexor reflex, windup of the flexor reflex to repeated stimuli was maintained during periods of increased sympathetic activity. This paradoxical depression of flexor reflexes and minimal effect on windup is consistent with inhibition of afferent feedback within the superficial dorsal horn. The results of this study bring attention to the possible interaction of motor and sympathetic reflexes in SCI above and below the T5 spinal level, and have implications for clinicians in spasticity management and for researchers investigating motor reflexes post SCI.

Intrathecal Nicorandil and Small-Dose Morphine Can Induce Spastic Paraparesis After a Noninjurious Interval of Spinal Cord Ischemia in the Rat

We investigated the interaction between nicorandil, a K(+)ATP channel opener, and morphine on motor function after a noninjurious interval of spinal cord ischemia in the rat. Spinal ischemia was induced by aortic occlusion for 6 min with a balloon catheter in Sprague-Dawley rats. All animals received intrathecal (IT) injection of morphine (1-60 microg) 1 h after ischemia. In addition to IT injection of morphine, group M (control), group MN (combination of morphine and nicorandil), and group MNG (combination of morphine, nicorandil, and glibenclamide) received IT saline, nicorandil (10 microg), and both glibenclamide (10 microg) and nicorandil (10 microg) after 150 min of reperfusion, respectively. A quantal bioassay for the effect of IT morphine on neurological function after ischemia was performed to calculate 50% effective dose values (ED50) for inducing paraparesis at 3 h of reperfusion. The ED50 in group M and group MN was 15.1 +/- 4.9 microg and 2.9 +/- 1.0 microg of IT morphine, respectively (P < 0.05). In Group MNG, the dose-response curve shifted back to the right and the ED50 for inducing paraparesis was 11.6 +/- 4.7 microg of IT morphine. The present study demonstrates that IT small-dose morphine combined with nicorandil induces spastic paraparesis after noninjurious interval of spinal cord ischemia in the rat.

Ketamine-Induced Potentiation of Morphine Analgesia in Rat Tail-Flick Test: Role of Opioid-, .ALPHA.2-Adrenoceptors and ATP-Sensitive Potassium Channels

Ketamine is known to improve opioid efficacy, reduce postoperative opioid requirement and oppose opioid associated pain hypersensitivity and tolerance. However, the mechanisms underlying these beneficial effects are not clear. This study investigated the effects of ketamine at a non-analgesic dose (30 mg/kg, i.p.) on analgesia induced by morphine (2.5, 5.0, 7.5 mg/kg, s.c.), using rat tail-flick test as an animal model of acute pain. Further, the role of opioid-, alpha2-adrenoceptors and ATP-sensitive potassium channels was examined on the potentiating effect of ketamine. Male rats received morphine alone at 5.0 and 7.5 but not at 2.5 mg/kg showed a dose-related increase in tail-flick latencies. The combination of morphine and ketamine resulted in dose-related increase in morphine analgesia, both on the intensity as well as on duration. The ketamine-induced potentiation of morphine (7.5 mg/kg) analgesia was unaffected by glibenclamide (3 mg/kg, s.c.) and only partially blocked by yohimbine (2 mg/kg, i.p.), but more completely abolished by naloxone (2 mg/kg, i.p.). Both morphine (5.0 mg/kg) and ketamine (30 mg/kg) alone did not evoke catalepsy in rats but on combination produced a synergistic effect, which was however, abolished by naloxone pretreatment. In the open-field test, while morphine (5.0 mg/kg) caused a depressant effect, ketamine (30 mg/kg) enhanced the locomotor activity. Nevertheless, in combination potentiated the morphine's depressant effect on locomotion, which was also antagonized by naloxone. These results indicate that ketamine at a non-analgesic dose can potentiate morphine analgesia, induce catalepsy and cause locomotor depression, possibly involving an opioid mechanism. This potentiation, although favorable in acute pain management, may have some adverse clinical implications.

The lower limb flexion reflex in humans

The flexion or flexor reflex (FR) recorded in the lower limbs in humans (LLFR) is a widely investigated neurophysiological tool. It is a polysynaptic and multisegmental spinal response that produces a withdrawal of the stimulated limb and resembles (having several features in common) the hind-paw FR in animals. The FR, in both animals and humans, is mediated by a complex circuitry modulated at spinal and supraspinal level. At rest, the LLFR (usually obtained by stimulating the sural/tibial nerve and by recording from the biceps femoris/tibial anterior muscle) appears as a double burst composed of an early, inconstantly present component, called the RII reflex, and a late, larger and stable component, called the RIII reflex. Numerous studies have shown that the afferents mediating the RII reflex are conveyed by large-diameter, low-threshold, non-nociceptive A-beta fibers, and those mediating the RIII reflex by small-diameter, high-threshold nociceptive A-delta fibers. However, several afferents, including nociceptive and non-nociceptive fibers from skin and muscles, have been found to contribute to LLFR activation. Since the threshold of the RIII reflex has been shown to correspond to the pain threshold and the size of the reflex to be related to the level of pain perception, it has been suggested that the RIII reflex might constitute a useful tool to investigate pain processing at spinal and supraspinal level, pharmacological modulation and pathological pain conditions. As stated in EFNS guidelines, the RIII reflex is the most widely used of all the nociceptive reflexes, and appears to be the most reliable in the assessment of treatment efficacy. However, the RIII reflex use in the clinical evaluation of neuropathic pain is still limited. In addition to its nocifensive function, the LLFR seems to be linked to posture and locomotion. This may be explained by the fact that its neuronal circuitry, made up of a complex pool of interneurons, is interposed in motor control and, during movements, receives both peripheral afferents (flexion reflex afferents, FRAs) and descending commands, forming a multisensorial feedback mechanism and projecting the output to motoneurons. LLFR excitability, mediated by this complex circuitry, is finely modulated in a state- and phase-dependent manner, rather as we observe in the FR in animal models. Several studies have demonstrated that LLFR excitability may be influenced by numerous physiological conditions (menstrual cycle, stress, attention, sleep and so on) and pathological states (spinal lesions, spasticity, Wallenberg's syndrome, fibromyalgia, headaches and so on). Finally, the LLFR is modulated by several drugs and neurotransmitters. In summary, study of the LLFR in humans has proved to be an interesting functional window onto the spinal and supraspinal mechanisms of pain processing and onto the spinal neural control mechanisms operating during posture and locomotion.

K(ATP) channel therapeutics at the bedside

The family of potassium channel openers regroups drugs that share the property of activating adenosine triphosphate-sensitive potassium (K(ATP)) channels, metabolic sensors responsible for adjusting membrane potential-dependent functions to match cellular energetic demands. K(ATP) channels, widely represented in metabolically-active tissue, are heteromultimers composed of an inwardly rectifying potassium channel pore and a regulatory sulfonylurea receptor subunit, the site of action of potassium channel opening drugs that promote channel activity by antagonizing ATP-induced pore inhibition. The activity of K(ATP) channels is critical in the cardiovascular adaptive response to stress, maintenance of neuronal electrical stability, and hormonal homeostasis. Thereby, K(ATP) channel openers have a unique therapeutic spectrum, ranging from applications in myopreservation and vasodilatation in patients with heart or vascular disease to potential clinical use as bronchodilators, bladder relaxants, islet cell protector, antiepileptics and promoters of hair growth. While the current experience in practice with potassium channel openers remains limited, multitude of ongoing investigations aims at defining the benefit of this emerging family of therapeutics in diverse disease conditions associated with metabolic distress.

Potassium channels and pain: present realities and future opportunities

Four families of potassium channels with different structures, functional characteristics and pharmacological sensitivity, are distinguished in neurons: voltage-gated (K(v)), calcium-activated (K(Ca)), inward rectifier (K(ir)) and two-pore (K(2P)) K(+) channels. During the last 15 years, numerous studies have demonstrated that the opening of some of these K(+) channels plays an important role in the antinociception induced by agonists of many G-protein-coupled receptors (alpha(2)-adrenoceptors, opioid, GABA(B), muscarinic M(2), adenosine A(1), serotonin 5-HT(1A) and cannabinoid receptors), as well as by other antinociceptive drugs (nonsteroidal antiinflammatory drugs [NSAIDs], tricyclic antidepressants, etc.) and natural products. Several specific types of K(+) channels are involved in antinociception. The most widely studied are the ATP-sensitive K(+) channels (K(ATP)), members of the K(ir) family, which participate in the antinociception induced by many drugs that activate them in both the central and the peripheral nervous system. The opening of G-protein-regulated inwardly rectifying K(+) channels (GIRK or K(ir)3), K(v)1.1 and two types of K(Ca) channels, the small- and large-conductance calcium-activated K(+) channels (SK and BK channels, respectively), also play a role in the antinociceptive effect of different drugs and natural products. Recently, drugs that open K(+) channels by direct activation (such as openers of neuronal K(v)7 and K(ATP) channels) have been shown to produce antinociception in models of acute and chronic pain, which suggests that other neuronal K(+) channels (e.g. K(v)1.4 channels) may represent an interesting target for the development of new K(+) channel openers with antinociceptive effects.

Pharmacological evidence for the activation of K(+) channels by diclofenac

The involvement of K(+) channels in the antinociceptive action of diclofenac was assessed in the formalin test. Local administration of diclofenac produced a dose-dependent antinociceptive effect due to a local action because drug administration in the contralateral paw was ineffective. Pretreatment of the injured paw with glibenclamide and tolbutamide (ATP-sensitive K(+) channel inhibitors), charybdotoxin and apamin (large- and small-conductance Ca(2+)-activated K(+) channel blockers, respectively), 4-aminopyridine or tetraethylammonium (voltage-dependent K(+) channel inhibitors) prevented diclofenac-induced antinociception. Given alone, K(+) channel inhibitors did not modify formalin-induced nociceptive behavior. Pinacidil (an ATP-sensitive K(+) channel opener) also produced antinociception which was blocked by glibenclamide. The peripheral antinociceptive effect of morphine (positive control) was blocked by glibenclamide and 4-aminopyridine but not by charybdotoxin or apamin. The results suggest that the peripheral antinociceptive effect of diclofenac may result from the activation of several types of K(+) channels, which may cause hyperpolarization of peripheral terminals of primary afferents.

Potassium channel openers: therapeutic potential in cardiology and medicine

Potassium (K(+)) channel openers (KCOs) define a class of chemically diverse agents that share a common molecular target, the metabolism-regulated ATP-sensitive K(+) (K(ATP)) channel. In view of the unique function that K(ATP) channels play in the maintenance of cellular homeostasis, this novel class of ion channel modulators adds to existent pharmacotherapy with potential in promoting cellular protection under conditions of metabolic stress. Indeed, experimental studies have demonstrated broad therapeutic potential for KCOs, including roles as cardioprotective agents, vasodilators, bronchodilators, bladder relaxants, anti-epileptics, insulin secretagogues and promoters of hair growth. However, clinical experience with these drugs is limited and their place in patient management needs to be fully established.

Participation of the nitric oxide-cyclic GMP-ATP-sensitive K(+) channel pathway in the antinociceptive action of ketorolac

The involvement of nitric oxide (NO), cyclic GMP and ATP-sensitive K(+) channels in the antinociceptive effect of ketorolac was assessed using the formalin test in the rat. Local administration of ketorolac in a formalin-injured paw produced a dose-dependent antinociceptive effect due to a local action, as drug administration in the contralateral paw was ineffective. Pretreatment of the injured paw with N(G)-L-nitro-arginine methyl ester (L-NAME, an NO synthesis inhibitor), 1H-(1,2,4)-oxadiazolo(4,2-a)quinoxalin-1-one (ODQ, a soluble guanylyl cyclase inhibitor) or glibenclamide (an ATP-sensitive K(+) channel blocker) prevented ketorolac-induced antinociception. However, pretreatment with saline or N(G)-D-nitro-arginine methyl ester (D-NAME) did not block antinociception. Local administration of S-nitroso-N-acetylpenicillamine (SNAP, an NO donor) was inactive by itself, but increased the effect of ketorolac. The present results suggest that the antinociceptive effect of ketorolac involves activation of the NO-cyclic GMP pathway, followed by an opening of ATP-sensitive K(+) channels at the peripheral level.

Norepinephrine modulates single hypothalamic arcuate neurons via alpha(1)and beta adrenergic receptors

The effects of norepinephrine (NE) on the electrophysiological activities of single hypothalamic arcuate neurons were studied using extracellular recording of 385 neurons from 169 brain slices in rats. The results showed that: (1) of 236 neurons selected randomly and tested with NE application, 137 (58.0%) were excited, 67 (28.4%) were inhibited, and 32 (13.6%) failed to respond; (2) substitution of low Ca(2+)-high Mg(2+) artificial cerebrospinal fluid (ACSF) for normal ACSF abolished the NE-induced inhibitory effect but failed to abolish the excitatory effect; (3) both the NE-induced excitatory and inhibitory effects were antagonized partly by phentolamine, prazosin, and propranolol but not by yohimbine; (4) naloxone and glibenclamide, a blocker of adenosine triphosphate-sensitive (K(ATP)) channels, blocked the NE-induced inhibitory effect; and (5) neurons that were inhibited by NE were also inhibited by morphine and cromakalim, an agonist of K(ATP) channels, and moreover, the morphine-induced inhibitory effect could be blocked by glibenclamide, while the cromakalim-induced inhibitory effect was not blocked by naloxone. These results imply that: (a) NE excites arcuate neurons through a mechanism that is insensitive to lowering the extracellular Ca(2+) suggesting a direct postsynaptic response through alpha(1)- and beta-adrenergic receptors, while NE inhibits cells through at least an inhibitory interneuron in arcuate and so is dependent on a Ca(2+)-sensitive presynaptic release mechanism; and (b) the inhibitory interneuron may be opioidergic, being excited first through alpha(1)- and beta-adrenergic receptors, after which the released opioids inhibit the neurons being recorded with an involvement of activation of K(ATP) channels. This possibility needs to be substantiated in much more detail.

Involvement of endogenous opioids and ATP-sensitive potassium channels in the mediation of apomorphine-induced antinociception at the spinal level: a study using EMG planimetry of flexor reflex in rats

The effects of intrathecally (i.t.) administered naloxone or glibenclamide, a blocker of adenosine triphosphate-sensitive potassium (KATP) channels, on the antinociception produced by i.t. apomorphine were observed by an integrated electromyogram measurement of hindlimb flexor reflex in lightly pentobarbital-anesthetized rats. The results showed that i.t. apomorphine produced a significant and dose-dependent antinociception and that the antinociception produced by i.t. apomorphine could be blocked dose dependently by i.t. naloxone or glibenclamide. The results suggest that endogenous opioids and ATP-sensitive potassium channels might be sequentially involved in the mediation of apomorphine-induced antinociception at the spinal level.

Transmitters involved in antinociception in the spinal cord

The possible physiological and pathophysiological role of monoamines-adrenergic transmitter (norepinephrine), serotonin; cholinergic transmitter (acetylcholine); inhibitory (gamma-aminobutyric acid) and excitatory (glutamate) amino acids; opioid and nonopioid peptides, enkephalins, beta-endorphin and substance P, neurokinin-A, neurokinin-B, neurotensin, cytokines, calcitonine gene-related peptide, galanin, neuropeptide Y, nerve growth factor, cholecystokinin; purines; nitric oxide; vanilloid receptor agonists (capasaicin); and nociceptin-in spinal transmission of pain is reviewed. The role of substance P, neurokinin-A and neurokinin-B in the dorsal horn has been identified. These were suggested to be primary afferent transmitters mediating or facilitating the expression of nociceptive inputs. Pronociceptive modulators will be discussed later. Recent findings showing that N-methyl-D-aspartate (NMDA) receptor activation generates nitric oxide and prostanoids that enhance pain transmission whereas adenosine release acts to control these NMDA-mediated events are also mentioned. The clinical importance of centrally acting alpha2-adrenoceptor agonists (clonidine and dexmedetomidine) is also discussed. Antinociceptive and morphine-potentiating drugs are ideal adjuvants for anesthesia; their application in spinal anesthesia is highlighted. The recent development in understanding the importance of noradrenergic transmission and subtypes of alpha2-adrenoceptors (alpha2A and alpha2B) for the first time is reviewed.

Adenosine mediates spinal norepinephrine-produced antinociception as revealed by nociceptive discharges in parafascicular neurons in rats

The effects of intrathecal pretreatment with aminophylline on intrathecal norepinephrine-produced or serotonin-produced suppression of noxiously evoked discharges in thalamic parafascicular neurons were investigated in 35 urethane-anesthetized rats. The results showed that: (1) both intrathecal norepinephrine (15 nmol) or serotonin (20 nmol) produced significant suppression of noxiously evoked discharges in parafascicular neurons; (2) intrathecal aminophylline (120 nmol) blocked the norepinephrine-produced suppression of noxiously evoked discharges, while the same dose of aminophylline exhibited no significant effect on the serotonin-produced suppression of these discharges in parafascicular neurons. The results suggest that spinal norepinephrine-produced, but not serotonin-produced, antinociceptive effects may be mediated by adenosine as one of successive chemical links in the spinal dorsal horn circuitry.

Endogenous opioids and ATP-sensitive potassium channels are involved in the mediation of apomorphine-induced antinociception at the spinal level: a behavioral study in rats

The effects of intrathecally (i.t.) administered glibenclamide, a blocker of adenosine triphosphate-sensitive potassium (K(ATP)) channels, or naloxone on the antinociception produced by i.t. apomorphine or morphine were observed and analyzed in rats by tail-flick (TF) test. The results showed that: (1) i.t. apomorphine produced a significant and dose-dependent antinociception, (2) the antinociception produced by i.t. apomorphine could be blocked dose-dependently by i.t. glibenclamide or naloxone, (3) the antinociception produced by i.t. morphine could also be blocked dose-dependently by i.t. glibenclamide. The results suggest that endogenous opioids and ATP-sensitive potassium channels might be involved in the mediation of apomorphine-induced antinociception at the spinal level.